As portable electronic products are developed rapidly, the demand for rechargeable batteries becomes increasingly larger. Rechargeable batteries include the conventional Ni—Ca battery, Ni—H battery, and Li-ion battery as well as the recently developed Li-polymer battery. The voltage provided by different type of rechargeable battery varies, and the required operating voltages of the portable electronic products are different. Therefore, battery manufacturers usually connect several batteries in series to produce a battery module for providing a voltage capable of meeting the operating voltage requirement of the portable electronic products.
When the electric power of batteries of a battery module is exhausted, it is necessary to recharge the battery by a battery charger for the next time of use. However, the battery capacity is different due to mismatch in manufacture and application. For example, a 7.4V lithium battery module is formed by connecting two pieces of 3.7V lithium batteries in series. For factory default setting, the electric power storage capacities of the two pieces of batteries are 80% and 70% respectively. Since the lithium batteries will be damaged if they are overcharged, therefore the lithium battery charger will stop charging the battery module as soon as any one of the lithium batteries therein is fully charged. Now, the electric power storage capacities of the two batteries are 100% (maximum level of a battery charge) and 90% respectively. The battery module can be used until the electric power storage capacity of any one of the batteries therein drops to 0% (minimum level of a battery discharge). Therefore, it is necessary recharge the battery module for a further use when the electric power storage capacities of the two batteries drop to 10% and 0% respectively.
From the description of the aforementioned example, the electric power storage capacities of batteries in a battery module are different, and thus the actual use of electric power storage capacity of the battery module is determined by the battery with the lowest electric power storage capacity. In addition to the variation of factory default electric power storage capacity of each battery of a battery module, a battery also self-discharges electric power, when the battery is not in use. Since each battery self-discharges electric power at a different rate, therefore an unbalanced electric power storage capacity will result among the batteries, and the usable capacity of the battery module is reduced as the using time of the battery increases, and the efficiency of using battery module becomes lower, and the using time of the battery module from fully charged to empty becomes shorter.
With reference to FIG. 1 for a schematic circuit diagram of a digital battery balancing controller disclosed in a datasheet of the Intersil ISL9208 product, a digital battery balancing controller 10 comprises a battery balance microprocessor 5 and a plurality of transistor switches S1-S7. The transistor switches S1-S7 are connected with a plurality of batteries BAT1-BAT7 in parallel through a plurality of resistors R1-R7 respectively. The voltage of the batteries BAT1-BAT7 is converted into a digital signal through an analog-to-digital converter (A/D converter), and the battery balancing controller 5 determines a higher-voltage battery by a built-in algorithm according to the digital signal of the voltage of the batteries BAT1-BAT7 and conducts the transistor switch connected in parallel to the higher-voltage battery, such that the charging current of each battery can be adjusted according to the voltage of each battery to achieve a balanced charging function. However, the voltage of each battery must be converted into a digital signal by an analog-to-digital converter before the digital battery balancing controller 5 processes, and the analog-to-digital converter increases the chip area 10 of the digital battery balancing controller 10 significantly and incurs a high cost. In addition, the digital battery balancing controller 5 is restricted by its design, such as the ISL9208 chip can support a battery module composed of 5 to 7 pieces of batteries only, and the scope of applicability is limited.